Brown — Developmental Stages in Streptelasma rectum. 277 



Art. XXV. — Developmental Stages in Streptelasma rectum, 

 Hall • by Thomas C. Brown. 



In the course of studies on the phylogeny of the Zaphren- 

 tidse, E. and H., my attention was attracted by the point 

 recently made by J. E. Duerden as to the primary septal con- 

 dition in rugose corals, whether tetrameral or hexameral. The 

 question is, are there only four primary septa in rugose corals 

 as suggested by Kunth and until recently accepted by the 

 majority of workers, and recently shown by Gordon for Strep- 

 telasma prof undum, or are there six primary septa as suggested 

 by Ludwig and the Count de Pourtales and claimed as defi- 

 nitely proven by Duerden. 



As this question involved the foundation on which all my 

 work must be based, I decided to investigate it. Duerden's 

 view seemed by far the most acceptable, as it brought the 

 rugose corals into line with all the other corals and zoanthids 

 as having an original hexamerous character. 



In Science for August 24, 1906, Duerden states that in six 

 species of Rugosa he has definitely proved the existence of 

 six primary septa. These species are Streptelasma rectum 

 Hall, Cyathaxonia cynodon E. and H., Hadrophyllum glans 

 (White), Hadrophyllum paticiradiatum E. and H., Microcy- 

 clus discus Meek and Worthen, and Lophophyllum proliferum 

 E. and H. One of the striking facts about- these six species is 

 that they all occur comparatively late in the geological distri- 

 bution of the Rugosa. Four are Devonic and two are Car- 

 bonic species, and of the four Devonic species three are almost 

 disc-like in shape and therefore present extreme difficulties 

 when one attempts to get the earliest stages. I therefore 

 selected the fourth Devonic species, Streptelasma rectum Hall, 

 to test the validity of Duerden's statements, and with somewhat 

 startling results. 



The collections in the Paleontological Laboratory of Colum- 

 bia University are particularly rich in this species, and from 

 some two or three hundred corallites I selected the most per- 

 fect individuals for the investigation. Some of these were 

 found with perfect tips and some with the end slightly frac- 

 tured. Following the method described by Duerden in his 

 latest paper in the Annals and Magazine of Natural History 

 for September, 1906, an individual corallite held by the large 

 calicular end was ground off at the tip very gradually on a 

 plate of glass with fine emery and each successive stage of 

 development was carefully noted and sketched. These suc- 

 cessive stages are enlarged and shown in the accompanying 



Am. Jour. Sci. — Fourth Series, Vol. XXIII, No. 136.— April, 1907. 

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